Metal parts, for example, those used in jet engines are generally required to meet very precise tolerances. Damage to metal parts during use or during machining where a part is overmachined can prevent the part from falling within the set tolerances and require that the part be repaired or replaced. To repair such damage, new metal must be brazed to the surface of the part.
The new metal needs to have a composition similar to the base metal or have properties consistent with the designed usage. Thus, diffusion braze fillers are combined with powder base metal to provide a composition which brazes to the base metal at a temperature lower than the melting point or softening point of the base metal.
Typically, this was done by forming a slurry which includes the powdered base metal, powdered diffusion braze filler and a binder which could be, for example, a methacrylate binder, an alginate binder or the like. These systems provide acceptable results. However, well defined geometries needed for some repairs were very difficult to obtain.
Further, slurries such as these are difficult to use. The binder system must be initially mixed. Then the precise amount of base metal and diffusion filler must be combined. This has a limited shelf life. It cannot be mass produced for sale and subsequent use if shelf life is a consideration (including settling of constituent powders). It is usually prepared by the actual user which creates the potential problem of human error.
Also, the boron typically used in the braze alloy could localize or puddle on the surface of the part. This weakens the base metal and can destroy the part. Slurries are also difficult to conform, resulting in poor ability for large build-up repairs.
A further improvement in making base metal repairs is disclosed in U.S. Pat. No. 5,952,042. This patent discloses formation of preforms from a blend of base metal powder, braze alloy powder and a polyfluorocarbon binder. This can be blended to form a tape which can be cut to size and presintered to form a preform that can then be placed on the worn area of a metal part. The metal part is then subjected to a thermal cycle which causes the preform to braze to the surface of the metal part.
This has been highly effective. However, the separate presintering process is time-consuming and adds to the expense of the preform. The presintering step does cause densification of the presintered preform. However, there is some slight shrinkage during the final brazing step. This can be compensated when forming the preform. However, it is preferable to eliminate all shrinkage.
A further method to add metal to the surface of a metal part is by thermal spraying. This has only limited applications because it can be very labor intensive, requiring a significant amount of masking in order to function properly as well as significant clean-up. Also it is difficult to precisely prepare the surface of the article which is to be sprayed and basically it is difficult to repeatedly ensure quality repairs.
The present invention is premised on the realization that a preform which can be easily bonded to a metal surface can be formed by thermal metal spraying a metal onto a substrate to form a preformed sheet. The metal can be a blend of a high melt alloy such as base metal powder or a coating powder with a low melt powder, preferably a braze alloy, a pure base metal powder (to be subsequently brazed or welded), or pure braze alloy where applicable. This preformed sheet is separated from the substrate forming a sheet. If the sheet is a blend of base alloy and braze alloy it can be cut into preforms and the preforms brazed directly onto a metal surface without use of additional braze alloys and the like. The low temperature braze alloy in the sheet will permit the sheet to melt at a lower temperature than the melting point of the high temperature and of course at a temperature lower than the melting temperature of the part being coated.
The preformed metal can be temporarily welded to the spot to provide accurate placement of the metal itself.
If the preformed sheet is the high temperature metal itself it can be bonded onto the surface of the part with a separate braze alloy or welded onto the part.
A variety of thermal spraying techniques can be used. One preferred method is high velocity oxygen fuel spraying. Other methods include wire arc spraying, flame spraying, air plasma spraying and the like. When necessary to form a dense sheet which easily separates from the support sheet, a two step process can be used. In the first step, a less aggressive thermal spraying technique is used to form a first layer. A more aggressive spraying technique is used to apply more metal to the first layer.